At present, people in some developed countries are well nourished, and said to be almost free from nutrient deficiency diseases. However, according to national nutrition examinations made every year in Japan, only calcium is not sufficiently taken with respect to the necessary intake. In order to supplement such an insufficient intake of calcium, it is on the one hand recommended to ingest calcium supply food such as dairy products, small fish and seaweeds, and on the other hand energetically attempted to develop various types of calcium preparations and calcium enriched food.
Many of minerals including calcium, however, are commonly sparingly absorptive, and hence mere enrichment of minerals in food can not be effective for a sufficient supply of minerals. Also, food enriched in only one kind of mineral may competitively inhibit absorption of other minerals, and rather has a possibility of causing a deficiency of minerals, as pointed out in a report (Science of Food, the November, 1989 Issue, pages 8-16).
Hence, in recent years, research and development has been carried out on various absorption promotors that promote absorption of minerals tending to be insufficiently taken, in particular, calcium. For example, casein phosphopeptide (CPP), which is a phosphoserine-containing peptide having about 20 amino acid units, is known to prohibit the insolubilization of calcium in the digestive organs to promote the absorption of calcium.
The CPP, however, is contained in casein only in an amount of about 1/50 of casein, and has had the problem that it is very expensive. The CPP is also a peptide having about 20 amino acid units. Hence, it has a possibility that its molecular structure necessary for the solubilization of calcium is destroyed when digested by proteolytic enzymes contained in gastric juice and intestinal juice and consequently no intended effect can be obtained.
Meanwhile, soybean protein is a protein composed chiefly of globulins such as 11S, 7S and 2.8S, and, different from .beta.-casein, contains almost no phosphoserine having a phosphoric acid group. Also, soybean protein contains phytic acid and salts thereof in a reasonable quantity. Such phytic acid and salts thereof are known to inhibit the intracorporeal absorption of minerals, in particular, of calcium (Toshiro Hayakawa, The First Niigata Prefecture Food Biotechnology Social Meeting, Separate Volume, 1989). As evidence therefor, it is reported that a comparative test made in respect of CPP and separated soybean protein to examine their calcium absorption promotion effect revealed that the CPP showed an apparent absorption of 60% but the separated soybean protein about 30% (Japan Nutrition Food Society, 45 (4) 333, 1992).
It is also reported that the phytic acid and salts in soybean protein can be removed to a content of as small as 0.14% by extracting soybean protein from a soybean protein-containing raw material with use of, e.g., a 8.5% by weight brine, and treating the resulting extract using an ultrafiltration membrane (Rham and Jost, J. Food Sci. 44 (2) 596, 1979).
In relation to the above finding, the present inventors have undertaken research on the phytic acid and salts thereof contained in soybean protein. In the course of the research, they have discovered that the phytic acid and salts thereof can be efficiently removed from a soybean protein-containing raw material by precipitation utilizing a calcium salt and/or a magnesium salt and electrodialysis, used in combination, and the soybean protein thus obtained, having the phytic acid and salts thereof in a low content, well effectively promotes the absorption of minerals. In this regard, they have filed patent applications, having been laid open as Japanese Patent Applications Laid-open No. 7-227215 and No. 7-228540.
However, when attempts have been made to obtain from soybean protein-containing raw materials soybean protein having a low level of phytic acid, difficulty has been encountered in removing the phytic acid. For example, in the above report by Rham and Jost, it is stated that phytic acid and salts thereof are removed when the brine is in a salt concentration of 8.5% but are not well removed when it is in a salt concentration of 7%. Thus, it is difficult to do so in the case when the brine (an extracting medium) has a low salt concentration. However, such extraction with a salt solution having a high concentration has the problem that it takes a long time for desalting and makes productivity poor, resulting in a high production cost.
Accordingly, the present inventors made further researches, and have discovered a process for producing soybean protein by extracting soybean protein from a soybean protein-containing raw material with use of an aqueous inorganic salt solution having a concentration not less than 1.0% by weight and less than 7.5% by weight, and subjecting the resulting extract to desalting by electrodialysis or ultrafiltration, followed by removal of phytic acid and salts thereof by isoelectric point precipitation. In this regard, they have filed a patent application as Japanese Patent Application No. 6-337321 (Japanese Patent Application Laid-open No. 8-173052).
According to the above process, even when the extracting medium has a relatively low salt concentration, the phytic acid and salts thereof can be well removed by the combination of electrodialysis or ultrafiltration with isoelectric point precipitation, and the desalting can be carried out in a shorter time. Hence, a soybean protein having phytic acid and salts thereof in a low content can be obtained while enjoying an economical advantage.
The above process enables extraction with a medium having a low salt concentration. However, it still requires the desalting treatment such as electrodialysis or ultrafiltration, and has had a problem of requiring plant investment therefor and treatment cost. Also, even though the salt concentration can be relatively low, there has been the problem that in an instance where bean-curd refuse, which is a residue formed after extraction of soybean protein from a soybean protein-containing raw material, is utilized as feed or the like the residue still has a high salt concentration and the salt must be removed.